If you are new to SiteMesh browse over to Getting Started with SiteMesh which explains how to setup SiteMesh in your web application.
SiteMesh build and source code are available at GitHub. The jar files are also located on the Maven Repository.
Current Stable Release
Version 2.4.2
sitemesh-example.war - a pre-packaged web project with many examples of SiteMesh in action.
Past Releases
Version 2.4.1
Version 2.4
Version 2.3
Version 2.2.1
Version 2.0.1